Gas phase folding of an (Ala)(4) neutral peptide chain: spectroscopic evidence for the formation of a beta-hairpin H-bonding pattern

IR and UV laser spectroscopy of an Ala-based 4-residue model peptide recorded under gas phase isolated conditions provides evidence for the intrinsic stability of compact folded structures resembling the extremity of a beta-hairpin, with a C-14 H-bond bridging the two ends of the chain, and enables us to assess the capabilities of new quantum chemistry techniques to account for dispersive interactions in a medium-size molecule.